1. Field of the Invention: This invention relates to satellite control systems and more particularly, to a system for the on-orbit correction of yaw error by automatic means using minimal hardware.
2. Description of the Prior Art: U.S. Pat. No. 3,866,025 uses on board computer 26 in conjunction with rate gyros in a satellite control system. The present invention uses electronic means in lieu of heavy, bulky gyros to estimate yaw angle from roll angle and yaw momentum.
U.S. Pat. No. 3,998,409 uses star sensors, sun sensors and gyroscopes to perform attitude control. These bulky items are not necessary in the present invention. The patent covers a zero momentum system unlike the momentum storage satellite that is described in the present specification.
U.S. Pat. No. 3,999,729 is a zero momentum system unlike the stored momentum system of the present specification.
U.S. Pat. Nos. 4,032,759 and 4,046,341 show a Kalman filter as a shipboard reference for an aircraft navigation system. The present invention uses a Luenberger observer, which is somewhat related to a Kalman filter, in a satellite on-orbit correction loop.
U.S. Pat. No. 4,071,211 utilizes sun sensors and knowledge of ephemeris to estimate yaw, unlike the present invention which does not require either.
U.S. Pat. No. 4,106,094 is a general description of estimators in inertial navigation systems.
Bryson and Luenberger, "The Synthesis of Regulator Logic Using State-Variable Concepts", Proceedings of the IEEE, Vol. 58, No. 11, November 1970 at 1803, provides a tutorial introduction to estimation theory and the type of filtering which became known as a Luenberger observer.
Spector and Iwens, "Attitude Control of a Comunication Satellite During Stationkeeping Using a Yaw Estimator", published Aug. 11, 1980, at the Guidance and Control Conference of the American Institute of Aeronautics and Astronautics, Danvers, Mass., subsequent to the completed conception of the present invention, is an application of Luenberger observers and Kalman filtering to a satellite during stationkeeping. The present invention, on the other hand, is an on-orbit attitude determination and control system. The paper's short term (on the order of 600 seconds) yaw correction loop is designed to counteract high disturbance torques caused by thruster firing, whereas the present invention's slow loop (on the order of an orbit period of 24 hours) counteracts low torques, such as those caused by solar pressure. The paper's detailed derivation addresses a system with momentum along the pitch axis only, due to a single pitch momentum wheel, whereas the present specification covers the embodiment of momentum along both the pitch and yaw axes.
Terasaki, "Dual Reaction Wheel Control of Spacecraft Pointing", Air Force Report No. SAMSO-TR-68-18, November 1967, discloses a control loop in a spacecraft having two momentum wheels. The control loop controls the speed of one of the wheels to damp nutation, and is relevant to the fast loop portion of the present invention.
Lebsock, "High Pointing Accuracy With a Momentum Bias Attitude Control System", Paper 78-569 at the AIAA 7th Communications Satellite Systems Conference, San Diego, Calif., Apr. 24-27, 1978, submitted July 11, 1978, revision received Nov. 5, 1979, provides an overview of the satellite attitude control system for which the present invention had its initial implementation. The paper does not disclose details of the yaw error correction system of the present invention.
Bowers, Rodden, Scott, and DeBra, "Orbital Gyrocompassing Heading Reference", Journal of Spacecraft and Rockets, Vol. 5, No. 8, August 1968, pp. 903-910, discloses a gyrocompassing technique for estimating yaw angle from roll angle. It differs from the present invention in that it does not disclose a stored momentum system. It uses bulky gyroscopes, which the present invention eliminates. Further, the present invention gets better accuracy.
Secondary references are U.S. Pat. Nos. 3,350,033; 3,591,108; 4,010,921; and 4,062,509.